A weather radar allows location of precipitations such as rain, snow or hail, measurement of their intensity and possibly identification of dangerous phenomena. Most weather radar are installed on the ground and are often part of a larger weather surveillance network. But airborne applications are increasingly coming into being, air transport being particularly interested in meteorological phenomena. The concern is notably with bypassing cumulonimbi, enormous clouds much feared by pilots as they sometimes create violent storms. Even airliners change their route in order to avoid crossing the path of certain particularly threatening cumulonimbi. This is because the lightning, the hail and the strong wind shears inside the cloud add to the risk of ice accretion and may endanger the flight if the pilot attempts to pass through.
A weather radar allows the detection of extended voluminous targets that clouds are, of which it must provide the position, size and dangerousness. To do that, a weather radar may, for example, emit a wave in the X-band. The distance to a cloud is deduced from the time necessary for the emitted pulse to carry out a return trip from the radar antenna to the cloud at the speed of light. This time corresponds simply to the time between emitting a pulse and receiving its echo. Estimation of the size of a cloud entails estimating its extent, i.e. the maximum horizontal distance it extends over, and estimating its elevation, i.e. the maximum vertical distance it extends over. Estimation of the extent profits notably from the azimuth scan of the radar beam. Estimation of the elevation profits notably from the elevation scan of the radar beam. By way of an indication, the elevation of a cumulonimbus often exceeds 10000 meters! It is the elevation which primarily characterizes the dangerousness of the cloud, for the higher a convective cloud is, the more dangerous it is. But the danger level of the cloud is also linked with its reflectivity factor, designated Z, which characterizes the concentration of hydrometeors suspended in a volume of air, in liquid or solid form. In a way, the reflectivity factor Z represents the intensity of the cloud. Once in a logarithmic scale, it is represented in dBZ.
In concrete terms, a display console displays to the on-board crew a simplified representation of clouds based on elementary geometrical shapes such as parallelepipeds, the color of which characterizes the intensity, whether it involves rain, snow or hail. For example, the color black is often used for dry air, i.e. the absence of cloud. Green and yellow may be used for moderate humidity concentrations. Red is often used for areas with very high humidity concentration, i.e. the most dangerous areas which it is absolutely necessary to bypass.
The display must be almost instantaneous, possibly on several screens used by various crew members throughout the flight. This simplified graphical representation of clouds is constructed on the screen using previously made intensity measurements, these measurements being stored for a greater or lesser period in a suitable memory space. Access to the data must therefore be fast and efficient, which is not without difficulties when the large quantity of data to be stored and therefore the memory space to be addressed is considered. In fact, it involves still making an intensity value available for every position (x, y, z) of space within range of the radar! The data must therefore be stored in a structured manner in order to optimize access to them while avoiding notably accessing them sequentially. This is one of the technical problems to which the present invention proposes to provide an innovative solution.
One current solution consists in saving the intensity information in a 3D matrix with each dimension corresponding to a spatial dimension. Hence, a triplet of indices (i, j, k) corresponds to each position (x, y, z), the coefficient of the 3D matrix at the location (i, j, k) containing an intensity value associated with the position (x, y, z). But such a solution is costly in terms of memory space: whatever the quantity of relevant information saved, the purely spatial quantification in a matrix makes it necessary to reserve a large memory space. For the matrix may possibly be largely filled with zero or insignificant values over an entire area. Thus, the memory is needlessly burdened, notably by clear weather, which is particularly damaging in airborne systems, the resources of which are limited. Of course, the problem is accentuated when the display precision increases, i.e. when the surface covered by a coefficient (i, j, k) of the 3D matrix decreases while the weather area to be stored remains the same.